diff --git a/apps/desktop/electron/windows-sandbox-fallback.test.ts b/apps/desktop/electron/windows-sandbox-fallback.test.ts
new file mode 100644
index 000000000000..19206a496533
--- /dev/null
+++ b/apps/desktop/electron/windows-sandbox-fallback.test.ts
@@ -0,0 +1,208 @@
+import assert from 'node:assert/strict'
+import fs from 'node:fs'
+import os from 'node:os'
+import path from 'node:path'
+
+import { test } from 'vitest'
+
+import {
+  ALL_APPLICATION_PACKAGES_SID,
+  WINDOWS_SANDBOX_BREAKPOINT_EXIT,
+  WINDOWS_SANDBOX_MARKER_FILENAME,
+  alreadyHasNoSandbox,
+  buildIcaclsGrantArgs,
+  buildNoSandboxRelaunchArgs,
+  grantAllApplicationPackagesAcl,
+  isWindowsSandboxBreakpointExit,
+  markerAfterSuccessfulBoot,
+  nextSandboxMarkerAfterLaunchDecision,
+  parseSandboxMarker,
+  readSandboxMarker,
+  sandboxMarkerPath,
+  shouldEnableWindowsNoSandbox,
+  shouldRelaunchForGpuSandboxCrash,
+  writeSandboxMarker
+} from './windows-sandbox-fallback'
+
+test('isWindowsSandboxBreakpointExit recognizes signed and unsigned STATUS_BREAKPOINT', () => {
+  assert.equal(isWindowsSandboxBreakpointExit(WINDOWS_SANDBOX_BREAKPOINT_EXIT), true)
+  assert.equal(isWindowsSandboxBreakpointExit(-2147483645), true)
+  assert.equal(isWindowsSandboxBreakpointExit(0x80000003), true)
+  assert.equal(isWindowsSandboxBreakpointExit(1), false)
+  assert.equal(isWindowsSandboxBreakpointExit('nope'), false)
+})
+
+test('alreadyHasNoSandbox honors argv and ELECTRON_DISABLE_SANDBOX', () => {
+  assert.equal(alreadyHasNoSandbox(['--foo', '--no-sandbox'], {}), true)
+  assert.equal(alreadyHasNoSandbox([], { ELECTRON_DISABLE_SANDBOX: '1' }), true)
+  assert.equal(alreadyHasNoSandbox([], { ELECTRON_DISABLE_SANDBOX: 'true' }), true)
+  assert.equal(alreadyHasNoSandbox(['--disable-gpu'], {}), false)
+})
+
+test('shouldEnableWindowsNoSandbox stays off outside Windows and on clean markers', () => {
+  assert.deepEqual(
+    shouldEnableWindowsNoSandbox({ platform: 'linux', marker: { state: 'booting' } }),
+    { enable: false, reason: null }
+  )
+  assert.deepEqual(
+    shouldEnableWindowsNoSandbox({ platform: 'win32', marker: { state: 'ok' }, argv: [], env: {} }),
+    { enable: false, reason: null }
+  )
+  assert.deepEqual(
+    shouldEnableWindowsNoSandbox({ platform: 'win32', marker: null, argv: [], env: {} }),
+    { enable: false, reason: null }
+  )
+})
+
+test('shouldEnableWindowsNoSandbox recovers from uncleared boot and sticky fallback', () => {
+  assert.deepEqual(
+    shouldEnableWindowsNoSandbox({
+      platform: 'win32',
+      marker: { state: 'booting' },
+      argv: [],
+      env: {}
+    }),
+    { enable: true, reason: 'uncleared-boot-marker' }
+  )
+  assert.deepEqual(
+    shouldEnableWindowsNoSandbox({
+      platform: 'win32',
+      marker: { state: 'fallback' },
+      argv: [],
+      env: {}
+    }),
+    { enable: true, reason: 'sticky-fallback' }
+  )
+  assert.deepEqual(
+    shouldEnableWindowsNoSandbox({
+      platform: 'win32',
+      marker: { state: 'ok' },
+      argv: ['--no-sandbox'],
+      env: {}
+    }),
+    { enable: true, reason: 'already-enabled' }
+  )
+})
+
+test('marker transitions preserve sticky fallback after a successful recovered boot', () => {
+  assert.deepEqual(nextSandboxMarkerAfterLaunchDecision({ enabledNoSandbox: false }), {
+    state: 'booting'
+  })
+  assert.deepEqual(nextSandboxMarkerAfterLaunchDecision({ enabledNoSandbox: true }), {
+    state: 'fallback'
+  })
+  assert.deepEqual(markerAfterSuccessfulBoot({ fallbackActive: false }), { state: 'ok' })
+  assert.deepEqual(markerAfterSuccessfulBoot({ fallbackActive: true }), { state: 'fallback' })
+})
+
+test('sandbox marker round-trips through the userData file', () => {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'hermes-sandbox-marker-'))
+
+  try {
+    assert.equal(sandboxMarkerPath(dir), path.join(dir, WINDOWS_SANDBOX_MARKER_FILENAME))
+    assert.equal(readSandboxMarker(dir), null)
+
+    writeSandboxMarker(dir, { state: 'booting' })
+    assert.deepEqual(readSandboxMarker(dir), { state: 'booting' })
+    assert.equal(parseSandboxMarker({ state: 'fallback' })?.state, 'fallback')
+    assert.equal(parseSandboxMarker({ state: 'nope' }), null)
+  } finally {
+    fs.rmSync(dir, { recursive: true, force: true })
+  }
+})
+
+test('buildIcaclsGrantArgs targets ALL APPLICATION PACKAGES with inherited RX', () => {
+  assert.deepEqual(buildIcaclsGrantArgs('C:\\Hermes\\win-unpacked'), [
+    'C:\\Hermes\\win-unpacked',
+    '/grant',
+    `*${ALL_APPLICATION_PACKAGES_SID}:(OI)(CI)(RX)`,
+    '/T',
+    '/C',
+    '/Q'
+  ])
+})
+
+test('grantAllApplicationPackagesAcl is a no-op off Windows and reports exec failures', () => {
+  assert.deepEqual(grantAllApplicationPackagesAcl('C:\\x', { platform: 'darwin' }), { ok: false })
+
+  const calls: Array<{ file: string; args: readonly string[] }> = []
+  const ok = grantAllApplicationPackagesAcl('C:\\Hermes', {
+    platform: 'win32',
+    execFileSync(file, args) {
+      calls.push({ file, args })
+
+      return Buffer.alloc(0)
+    }
+  })
+
+  assert.deepEqual(ok, { ok: true })
+  assert.equal(calls.length, 1)
+  assert.equal(calls[0]?.file, 'icacls')
+  assert.deepEqual(calls[0]?.args, buildIcaclsGrantArgs('C:\\Hermes'))
+
+  const failed = grantAllApplicationPackagesAcl('C:\\Hermes', {
+    platform: 'win32',
+    execFileSync() {
+      throw new Error('access denied')
+    }
+  })
+
+  assert.equal(failed.ok, false)
+  assert.match(String(failed.error), /access denied/)
+})
+
+test('shouldRelaunchForGpuSandboxCrash only fires once for GPU breakpoint deaths', () => {
+  assert.equal(
+    shouldRelaunchForGpuSandboxCrash({
+      platform: 'win32',
+      details: { type: 'GPU', exitCode: WINDOWS_SANDBOX_BREAKPOINT_EXIT },
+      alreadyNoSandbox: false,
+      relaunchAttempted: false
+    }),
+    true
+  )
+  assert.equal(
+    shouldRelaunchForGpuSandboxCrash({
+      platform: 'win32',
+      details: { type: 'GPU', exitCode: WINDOWS_SANDBOX_BREAKPOINT_EXIT },
+      alreadyNoSandbox: true,
+      relaunchAttempted: false
+    }),
+    false
+  )
+  assert.equal(
+    shouldRelaunchForGpuSandboxCrash({
+      platform: 'win32',
+      details: { type: 'GPU', exitCode: WINDOWS_SANDBOX_BREAKPOINT_EXIT },
+      alreadyNoSandbox: false,
+      relaunchAttempted: true
+    }),
+    false
+  )
+  assert.equal(
+    shouldRelaunchForGpuSandboxCrash({
+      platform: 'win32',
+      details: { type: 'renderer', exitCode: WINDOWS_SANDBOX_BREAKPOINT_EXIT },
+      alreadyNoSandbox: false,
+      relaunchAttempted: false
+    }),
+    false
+  )
+  assert.equal(
+    shouldRelaunchForGpuSandboxCrash({
+      platform: 'linux',
+      details: { type: 'GPU', exitCode: WINDOWS_SANDBOX_BREAKPOINT_EXIT },
+      alreadyNoSandbox: false,
+      relaunchAttempted: false
+    }),
+    false
+  )
+})
+
+test('buildNoSandboxRelaunchArgs appends a single --no-sandbox flag', () => {
+  assert.deepEqual(buildNoSandboxRelaunchArgs(['--foo', '--no-sandbox', 'hermes://x']), [
+    '--foo',
+    'hermes://x',
+    '--no-sandbox'
+  ])
+})